Fibers stiffened in substantially individualized form and various methods for making such fibers have been described in the art. Individualized, stiffened fibers are generally regarded as being useful in absorbent product applications. Two primary categories of such fibers are individualized, crosslinked fibers and individualized, resin-treated fibers. The term "individualized, crosslinked fibers", refers to cellulosic fibers that have primarily intrafiber chemical crosslink bonds. That is, the crosslink bonds are primarily between cellulose molecules of a single fiber, rather than between cellulose molecules of separate fibers. Generally, monomeric crosslinking agents are contemplated for crosslinking of individualized fibers, although some resins that have been used to stiffen fibers are known to also have functionalities which may be useful for forming crosslink bonds. For the purpose of this discussion, fibers stiffened with such resins will be included in the term "resin-treated fibers." In general, three categories of processes have been reported for making individualized, crosslinked fibers. These processes, described below, are herein referred to as (1) dry crosslinking processes, (2) aqueous solution crosslinking processes, and (3) substantially non-aqueous solution crosslinking processes. The fibers themselves and absorbent structures containing individualized, crosslinked fibers generally exhibit an improvement in at least one significant absorbency property relative to conventional, uncrosslinked fibers. Often, this improvement in absorbency is reported in terms of absorbent capacity. Additionally, absorbent structures made from individualized crosslinked fibers generally exhibit increased wet resilience and increased dry resilience relative to absorbent structures made from uncrosslinked fibers. The term "resilience" shall hereinafter refer to the ability of pads made from cellulosic fibers to return toward an expanded original state upon release of a compressional force. Dry resilience specifically refers to the ability of an absorbent structure to expand upon release of compressional force applied while the fibers are in a substantially dry condition. Wet resilience specifically refers to the ability of an absorbent structure to expand upon release of compressional force applied while the fibers are in a moistened condition. For the purposes of this invention and consistency of disclosure, wet resilience shall be observed and reported for an absorbent structure moistened to saturation.
Processes for making individualized, crosslinked fibers with dry crosslinking technology are described in U.S. Pat. No. 3,224,926 issued to L. J. Bernardin on Dec. 21, 1965 and U.S. Pat. No. 3,440,135, issued to R. Chung on Apr. 22, 1969. Individualized, crosslinked fibers are produced by impregnating swollen fibers in an aqueous solution with crosslinking agent, dewatering and defiberizing the fibers by mechanical action, and drying the fibers at elevated temperature to effect crosslinking while the fibers are in a substantially individual state. The fibers are inherently crosslinked in an unswollen, state as a result of being dehydrated prior to crosslinking. Processes as exemplified in U.S. Pat. No. 3,224,926 and wherein crosslinking is caused to occur while the fibers are in an unswollen, collapsed state, are referred to as processes for making "dry crosslinked" fibers. Dry crosslinked fibers have been characterized by low water retention values (WRV).
Processes for producing aqueous solution crosslinked fibers are disclosed, for example, in U.S. Pat. No. 3,241,553, issued to F. H. Steiger on Mar. 22, 1966. Individualized, crosslinked fibers are produced by crosslinking the fibers in an aqueous solution containing a crosslinking agent and a catalyst. Fibers produced in this manner are hereinafter referred to as "aqueous solution crosslinked" fibers. Due to the swelling effect of water on ellulosic fibers, aqueous solution crosslinked fibers are crosslinked while in a swollen state. Relative to dry crosslinked fibers, aqueous solution crosslinked fibers as disclosed in U.S. Pat. No. 3,241,553 have greater flexibility and less stiffness, and are characterized by higher water retention value (WRV). Absorbent structures made from aqueous solution crosslinked fibers exhibit lower wet and dry resilience than pads made from dry crosslinked fibers.
In U.S. Pat. No. 4,035,147, issued to S. Sangenis, G. Guiroy and J. Quere on July 12, 1977, a method is disclosed for producing individualized, crosslinked fibers by contacting dehydrated, nonswollen fibers with crosslinking agent and catalyst in a substantially nonaqueous solution which contains an insufficient amount of water to cause the fibers to swell. Crosslinking occurs while the fibers are in this substantially nonaqueous solution. This type of process shall hereinafter be referred to as a nonaqueous solution crosslinking process; and the fibers thereby produced, shall be referred to as nonaqueous solution crosslinked fibers. The nonaqueous solution crosslinked fibers disclosed in U.S. Pat. No. 4,035,147 are alleged to not swell even upon extended contact with solutions known to those skilled in the art as swelling reagents. Like the dry crosslinked fibers discussed above, such fibers would be highly stiffened by crosslink bonds, and absorbent structures made therefrom would exhibit relatively high wet and dry resilience.
Crosslinked fibers as described above are believed to have application to lower density absorbent products such as diapers and also higher density absorbent products such as sanitary napkins. However, such fibers have not attained commercial significance. One reason for the lack of commercial success may be that dry crosslinked fibers in general and many nonaqueous solution crosslinked fibers have been characterized in the literature by excessive stiffness and dry resiliency. Such fibers are difficult to form into densified sheets for transport and subsequently refluff without fiber damage. Furthermore, when compressed in a dry state, pads made from these fibers exhibit a low responsiveness to wetting. Once they are subjected to sufficient pressure to provide a dry pad of stable, high density, the pads have reduced susceptibility to expand toward their precompression volume upon wetting. It is believed that this lack of responsiveness to wetting is due to excessive stiffness of the fibers and fiber breakage upon exposure to high levels of compression.
Commercial viability of nonaqueous solution crosslinked fibers in particular is severely hampered because of high capital costs of implementing such processes and because of the additional expense of solvents necessary for the extraction and reaction mediums.
Aqueous solution crosslinked fibers, while useful for certain higher density absorbent pad applications such as tampons wherein densities ordinarily are about 0.40 g/cc, are excessively flexible when in a wet state and therefore result in absorbent structures which have low wet resilience. Furthermore, upon wetting, aqueous solution crosslinked fibers become too flexible to structurally support the pad at lower fiber densities. The wetted pad therefore has limited ability to retain its volume or to expand upon wetting when in a compressed state and final absorbent capacity is reduced.
It is an object of this invention to provide commercially viable individualized, stiffened fibers and absorbent structures made from such fibers wherein the absorbent structures made from the stiffened fibers have high absorbency, wicking ability, wet resilience and responsiveness to wetting.
It is further an object of this invention to provide fibers and absorbent structures having the attributes described in the preceding paragraph in combination with sufficiently low dry resilience such that the absorbent structures can be easily compressed in a dry, volume-stable form which expands upon wetting.
Recently, thinness of absorbent products especially in the diaper and catamenial industries has become a highly desirable product attribute. Good absorbent performance is still an important aspect of such products. To date, good absorbency has been achieved largely through use of polymeric gel forming materials. The effectiveness of polymeric gel forming materials may be limited by an absorbent structure's ability to transport fluid to the polymeric gelling material or to portions of the absorbent structure due to swelling of the polymeric gelling material. Therefore, it is another object of this invention to provide absorbent structures, and cellulose fibers useful for making such absorbent structures, which have small caliper relative to absorbent structures of conventional, unstiffened fibers but which have superior wicking ability and absorbent capacity.
Upon reading the present disclosure, other objectives and benefits provided by the present invention may presently or later become apparent to those skilled in the art.